JP2012509560A - A tomographic atom probe including an electro-optic generator for high-voltage electric pulses - Google Patents

Abstract

The present invention relates to the general field of tomographic atom probes, and in particular to a tomographic atom probe that uses electrical pulses applied to electrodes to achieve evaporation of the sample being analyzed.
In order to generate these electrical pulses, the tomographic atom probe according to the present invention includes a high voltage generator connected to the electrode by an electrical connection comprising a chip of semiconductor material. The probe also includes a light source that can be controlled to generate a light pulse that is applied to the semiconductor chip. Throughout the irradiation, the chip is made conductive, which makes the high voltage generator and the electrode in electrical contact so that a potential step is applied to the electrode. The probe also includes means for applying a voltage step of opposite amplitude to the previous step at the end of the time interval Δt ₀ , so that the electrode eventually receives a voltage pulse of period Δt _0. .
[Selection] Figure 4

Description

  The present invention relates to the general field of high voltage pulse generators. The present invention relates to a pulse generator used to cause evaporation of atoms from a material sample placed on a tomographic atom probe, particularly for the purpose of analyzing the material sample.

An essential feature of a tomographic atom probe is the value of mass resolution that can be obtained using this probe. The mass resolution of an atom probe is an essential characteristic that characterizes in particular: That is,
The ability of the probe to unambiguously separate the various mass peaks of different isotopes in the elements that make up the material being analyzed. This ability to distinguish is more valuable if it relates to adjacent mass peaks with very different amplitudes.
The ability of the probe to improve the measurement accuracy of the composition in the material being analyzed, for example an alloy, by reducing the incorporation of detection noise at the mass peak corresponding to the element of the material being analyzed.

  Mass resolution is expressed in the form of the ratio R = m / dm, where m represents the mass corresponding to the peak in question (ie to the element) and dm is associated with this given peak. It represents the width of the mass peak with respect to the amplitude. If m is equal to 28 and dm is equal to 0.28 for example h = 0.5 (half height width), then R = 100 at half height.

  Thus, the problem presented to the manufacturers of such probes that make up time-of-flight mass spectrometers is to obtain the best possible spectral resolution. Thus, when obtaining excellent mass resolution, the problem presented to the manufacturers of such probes that make up time-of-flight mass spectrometers is to obtain the best possible spectral resolution. As is well known, obtaining excellent mass resolution involves controlling the rate of movement of material particles from the sample (from which the material particles are generally separated in ionic form) to the detector. In other words, as is well known, the problem presented is that atoms that are evaporated in ionic form are separated from the sample with the same initial potential energy, and then these atoms can move to the detector at approximately the same speed. Is to do so.

  As is also well known, the input of potential energy required to separate several atoms from the atomic layer of the sample is performed by applying a high value voltage pulse (high voltage pulse) during that period. Is in the order of 1 nanosecond in the sample extraction zone (tip).

  This voltage and curvature at the edge of the sample is sufficient to generate an electric field that is strong enough to obtain the “field effect evaporation” phenomenon, an effect well known to those skilled in the art. In practice, however, the generation of a pulse that reaches the required theoretical potential level almost instantaneously, always maintains this level for a given time, and then terminates almost instantaneously as well. The current situation in the field constitutes a real problem. Thus, pulses with a generally parabolic shape at the top are generally generated, which leads to a decrease in mass resolution when such pulses are used in tomographic atom probes. Therefore, changing the amplitude of the pulse applied to the sample during evaporation and then into the first few nanometers in the trajectory of the emitted ions is not a single energy line, but rather a wide energy (velocity) ) Lead to the appearance of spectrum. Thus, the spectral resolution of the atom probe depends on its ability to generate square wave pulses with high amplitude and steep edges.

  FIGS. 1 and 2 are simulated mass histograms for the fault atom probe or “Watap” acronym “Wide angle tomographic atom probe”, where the simulated probe is 0. This dependence is shown by a mass histogram with a given flight distance of 11 m.

  FIG. 1 shows a typical and convenient case, taken as an example, in which the evaporation pulse is a square wave pulse with a plateau of 200 picoseconds (ps) and an edge of 50 picoseconds.

  Next, FIG. 2 shows a less convenient case, where the evaporation pulse is a square wave pulse with a plateau of 200 picoseconds and an edge of 1000 picoseconds.

  Both the presence of peaks 11, 21 having a certain width and the presence of more extended but lower level tails 12, 22 can be seen in the respective figures. Here, the peak corresponds to the evaporation generated when the evaporation pulse reaches its maximum level (pulse plateau), while the tail is now generated during the time interval corresponding to the leading and trailing edges. Corresponding to the evaporation performed, an energy loss of evaporated ions is observed.

  Thus, the relative amplitude and duration of the peak on the one hand and the tail on the other hand (the part of the pulse located after the peak) characterizes the resolution of the probe in question, and the larger the amplitude of the peak and its width It can be seen from FIG. 2 that knowing that the smaller the is, the higher the resolution of the probe is, the increase in the rise and fall times of the pulse leads to a decrease in the resolution. In other words, the closer the evaporation pulse is to a rectangular wave shape, the higher the resolution of the probe.

  To limit this problem, manufacturers have steep edges or very short rise and fall times (typically less than 100 picoseconds) for very short pulses (typically less than 500 picoseconds). Looking for a way to generate a constant amplitude plateau with Well known prior art has proposed various approaches for generating such high voltage pulses.

  For example, there is a high voltage pulse generator using a mercury wet relay. However, the repetition frequency of pulses generated by such a device with a relatively old design is very low (on the order of 100 Hertz) considering the desired characteristics, which makes the analysis of the sample relatively slow.

  Also, there are more recently designed semiconductor devices that can generate short-term pulses that can set the amplitude over a wide voltage range, typically between a voltage close to 0V and a voltage of 4 kV. It becomes possible. Furthermore, these devices make it possible to obtain repetition frequencies up to several tens of kilohertz.

  However, these performances come at the expense of rise and fall times, i.e. degradation of the gradient at the edges of the pulses thus generated. Thus, in current prior art, such devices allow short duration high voltage square wave pulses, ie steep edges (typically less than 100 picoseconds) and short maxima (typically 500 picoseconds). It is impossible to generate a pulse with a constant level.

  In addition, there are pulse generation systems that can generate pulses with lower amplitudes that have the desired time characteristics but are either fixed or difficult to adjust. Therefore, these systems are not or are not well suited for use in the range of tomographic atom probes as follows. In other words, it is a tomographic atom probe intended for the analysis of different materials, and each material has a range of tomographic atom probes that require generation of a pulse whose voltage value is proportional to the high voltage that polarizes the sample. Not suitable for use or not very suitable.

  Thus, known prior art devices are generally not satisfactory in the context of making atom probes with high mass resolution.

  The object of the present invention is to solve certain problems in the generation of high voltage pulses having the above characteristics and thus to improve the mass resolution of tomographic atom probes.

For this purpose, the invention relates to a tomographic atom probe having means for applying an electroevaporation pulse of amplitude V _p and duration Δt ₀ to a sample brought to potential V ₀ . According to the invention, these means include
- and it is in initial potential V _i, and configured and arranged to apply an electrical pulse to the sample electrode,
A voltage generator capable of generating the voltage required to generate a pulse of amplitude V _p , connected to the electrode by an electrical connection that can be opened or closed;
Means for closing the electrical connection at a given time τ to apply a voltage step V _p to the electrode, the means being close to the electrode and between the generator and the electrode And a first light source that emits a light pulse of wavelength λ ₁ to the semiconductor chip, the chip being irradiated with a light pulse of wavelength λ ₁ Means to become conductive and close the electrical connection, the conduction time being a function of the duration of the applied light pulse;
- After closing the electrical connections, the end of the time Delta] t _0, by applying a voltage step amplitude (-V _p) to the electrodes, and means for the electrode to the potential V _i, a voltage step said means configured to be applied at a time τ ′ approximately equal to τ;
Is included.

According to a particular embodiment of the tomographic atom probe according to the invention, after closing the circuit, at the end of time Δt ₀ , means for applying a voltage step of amplitude −V _p to the electrode chip the generator. connected to, a transmission line having a characteristic impedance Z _c, consists of a transmission line is terminated at the downstream electrode by impedance equal to Z _c.

According to this particular embodiment, the transmission line length L ₀ is determined by the value of the time interval in question Δt ₀ , which is equal to the duration of the electrical pulse to be generated.

According to another particular embodiment of the tomographic atom probe according to the invention, after closing the circuit, at the end of the time Δt ₀ , the means for applying a voltage step of amplitude −V _p to the electrode has the wavelength λ _{A second} light source that emits _two light pulses onto a semiconductor chip, the chip becoming insulative when it is irradiated with a light pulse of wavelength λ ₂ and opening an electrical connection Lies in the light source.

According to this particular embodiment, the time interval Δt ₀ between the emission of the optical pulse of wavelength λ ₁ and the emission of the optical pulse of wavelength λ ₂ determines the duration of the electrical pulse that is generated.

  According to a particular embodiment of the tomographic atom probe according to the invention, the electrode on which the electrical pulse is generated is located facing the sample end where evaporation is expected.

  According to another particular embodiment of the tomographic atom probe according to the invention, the electrode from which the electrical pulse is generated consists of the sample itself.

  The device according to the invention makes it possible to form electrical pulses, in particular high voltage pulses, having very short rise and fall times, typically on the order of a few picoseconds. This method also allows the maximum amplitude of this pulse to be controlled and kept constant throughout the duration of the voltage plate. This method also makes it possible to generate an electroevaporation pulse, i.e. an electroevaporation pulse whose particular shape allows to optimize the mass resolution of the atom probe and in particular its wide-angle deformation.

  The features and advantages of the present invention will be better understood from the following description, which uses a specific embodiment selected as a non-limiting example and refers to the accompanying figures. The present invention will be described.

2 shows two mass histograms illustrated as examples of resolution problems caused by existing atom probe electroevaporation pulse generators. 2 shows two mass histograms illustrated as examples of resolution problems caused by existing atom probe electroevaporation pulse generators. 1 shows a schematic diagram illustrating how electroevaporation pulses are generally generated in well-known prior art atom probes. Fig. 4 represents a schematic diagram similar to the diagram of Fig. 3, showing the general operating principle of the device according to the invention in connection with the preferred embodiment. 2 shows an equivalent circuit diagram of a sample tip / back electrode assembly in an atom probe. FIG. FIG. 2 shows an equivalent circuit diagram of a semiconductor chip serving as a controlled switch in a preferred embodiment of the device according to the invention. 1 shows an illustration relating to a first preferred alternative embodiment of the device according to the invention. 1 shows an illustration relating to a first preferred alternative embodiment of the device according to the invention. 1 shows an illustration relating to a first preferred alternative embodiment of the device according to the invention. 1 shows an illustration relating to a first preferred alternative embodiment of the device according to the invention. 2 shows an illustration relating to a second preferred alternative embodiment of the device according to the invention. 4 shows an illustration relating to a third preferred alternative embodiment of the device according to the invention.

As shown in the schematic diagram of FIG. 4, the electrical pulse generator according to the present invention first includes a voltage generator 41 that can generate a voltage required to generate a pulse of amplitude V _{2 at} the electrode 32. However, this pulse allows for the evaporation to take into account the potential to which the sample is applied. This voltage generator is connected to the electrode 32 by an electrical connection 46 that can be opened or closed.

The device according to the invention is also for applying a voltage step V ₂ to the electrode, to bring the connection 46 closed and open in order to bring the potential of the electrode initially equal to the value V ₁ to V ₁ + V ₂ . Means are included. Means for effecting closure of connection 46 include a controlled switch inserted in the electrical circuit between generator 41 and electrode 32 near the electrode, and means for controlling the switch; Is included.

  A preferred but not exclusive embodiment of these means consists in using a chip 42 of semiconductor material attached to a circuit connecting the high voltage generator 41 and the electrode 32 to which the electrical pulse is applied.

The means for actuating the switch constituted by the chip 42 of semiconductor material is a light source 43, for example a laser light source, which emits a short pulse 44 of high intensity typically in less than 100 picoseconds. The length λ _{1 of the} pulse emitted by the light source 43 is determined as a function of the semiconductor used to form the chip 42. Furthermore, the energy contained in each pulse, typically on the order of 10 microjoules, is sufficient to set the semiconductor chip in a conductive state.

  According to the present invention, the assembly is configured such that the tip 42 is illuminated by the light source 43. To that end, the light pulse 44 is applied to the semiconductor chip 42 either directly through the transparent window 45 or by an optical fiber or any other means.

  Therefore, by applying a light pulse of a predetermined period to the chip made of semiconductor material, the conductivity of the chip is naturally developed. Initially, the chip changes from an insulating state to a conductive state in a short time τ. Subsequently, the chip remains conductive throughout the entire light pulse. Next, after the light pulse has been extinguished, the chip gradually returns to an insulating state at the end of time τ '.

Setting a chip of a semiconductor material to a conductive state, leads to the appearance of the voltage step in the electrode, then the electrode is a voltage V _1. Conversely, the return of the chip to the non-conductive state leads to the appearance of the opposite voltage step at the electrode, at which time the electrode is brought to the inactive potential V ₀ . As described above, an electric pulse is generated in the electrode, and the period thereof depends on the period of the applied optical pulse and the characteristics of the semiconductor constituting the chip. Thus, such a pulse can advantageously be very short.

  However, it should be noted that with such a structure, the time τ 'for returning to the insulating state is substantially much greater than the time τ. Because, when the semiconductor material is simply irradiated, the time taken to generate free carriers therein is much less than the time τ ′ taken to remove these carriers when irradiation is completed. Because. Furthermore, this time t 'cannot be modified by any external means. Thus, although much shorter than that produced by commonly used means, the duration of the generated electrical pulses cannot be completely controlled.

  This is the reason for the following. That is to say, in spite of the fact that the time for the semiconductor material chip acting as a switch here to return to the insulating state cannot be completely determined, for the elements shown in FIG. This is why it incorporates additional means to allow full control over the width of the electrical pulses generated in

The operation of these means is as follows. That is, to make the electrode potential almost instantaneously V ₀ , the amplitude −V ₁ after the circuit is closed, ie, at the end of time Δt ₀ after the chip is set to a conductive state in the preferred embodiment. Is applied to the electrode. In this way, the period can be shortened (about 100 picoseconds) compared to the period of pulses generated by a conventional pulse generator, and the leading and trailing edges are very steep (several picoseconds). And it is possible to obtain electrical pulses having a period that is similar or even approximately equal.

  Note the following: That is, such means directly affects the potential applied to the electrode, but may advantageously be associated with a wide variety of means for effecting closing and opening of the connection 46. Thus, the advantages provided by using such means are not limited to the specific case of the preferred embodiments described above, i.e. when these means consist of a chip of semiconductor material and a light source that generates light pulses.

Thus, from a physical point of view, in a preferred embodiment of the device according to the invention, the high voltage pulse is transmitted by one or more chips 42 of a semiconductor material (silicon, germanium, gallium arsenide, etc.) with a direct or indirect gap. Generated. Such elements are known for their electro-optical properties, but become conductive for a short time under the influence of short and strong light pulses 44 coming from, for example, a femtosecond pulsed laser generator. Accordingly, the wavelength λ ₁ of the light pulse 44 is selected to generate free charge by photoconductivity in the semiconductor material. In other words, the energy of the emitted photons is determined to be larger than the material gap used.

  Thus, when this tip 42 of material is not irradiated, its electrical resistance remains high at around 100 megohms. Conversely, when the chip is illuminated by a strong light pulse 44 emitted by the light source 43, a large number of free charges or carriers are emitted within the chip 42 by the photoconductive effect. The chip then changes from several megohms to several ohms impedance in a very short time, typically less than 100 picoseconds. The chip then becomes conductive. The time that the device is conductive (conduction time) corresponds to the lifetime of the free carriers, but additionally, the thickness of the chip, the nature of the semiconductor material used, and the time or more generally received by the semiconductor Depends on light energy.

It is known that the shape of the semiconductor chip 42 is actually required to be able to set a voltage of several kilovolts between the two ends of the chip when the chip is in an insulated state (open switch), Selected to avoid reaching the breakdown voltage of the material. This breakdown voltage is, for example, a constant of a semiconductor material equal to 3 · 10 ⁵ V / cm for silicon and 4 · 10 ⁵ V / cm for gallium arsenide. Therefore, a sufficient thickness is necessary. Conversely, in order to be able to operate at high pulse generation frequencies, it is desirable for the chip to change very rapidly from an insulated state to a conductive state and from a conductive state to an isolated state. This requires the use of a thin chip 42. Thus, the choice of the thickness imparted to the chip 42 results in particular from a compromise between the function of the material used and the chip conduction time (photoinduced carrier recombination time), which depends in particular on the nature of the material.

は、典型的には数十ギガヘルツである。したがって、できるだけ一定した電圧プレートを有するパルスを生成できるようにするために、チップに印加される光ビームの強度を活用して抵抗Ｒ_Ｓを調整し、どんな寄生振動も抑制するようにすべきである。しかしながら、チップが導電性の場合に、非ゼロ抵抗を維持することによって、立ち上がり時間は、数ピコ秒だけ劣化される。 From the point of view of operation, the chip 42 itself constitutes the RLC type circuit shown in FIG. 5, but with respect to this circuit, the resistor R _S is when the chip is not illuminated (switch open state). It changes from several megohms to several ohms when the tip is illuminated (switch closed state). The capacitance C _S of typically about 1 picofarads is a function of the shape of the chip. Next, the inductance L _S is about several tens of picohenries. This chip therefore represents a resonant circuit, whose resonant frequency ν _S

Is typically tens of gigahertz. Therefore, in order to be able to generate pulses with as constant a voltage plate as possible, the intensity of the light beam applied to the chip should be utilized to adjust the resistance _RS to suppress any parasitic oscillations. is there. However, by maintaining non-zero resistance when the chip is conductive, the rise time is degraded by a few picoseconds.

  Thus, according to the present invention, the material chip 42 directly influences the formation of the electrical pulses generated, and as a result, unlike the generator 31 equipped in the conventional device whose principle is shown in FIG. The generator 41 is a DC voltage generator, ie a generator that emits a wide pulse carved by the switch constituted by the chip 42.

  Note that the conduction time of the chip 42 and the development of the conductivity of the chip during the conduction time determines the maximum width of the electrical pulse applied to the sample. This period is in particular a function of the duration of the light pulse 44 and the nature of the chip material. Furthermore, the minimum rise time of the pulse is in turn a function of the structure of the electronic circuit in which the device is inserted inside, on the one hand, the free carrier generation rate. Thus, by adopting an appropriate structure, a minimum rise time of about several picoseconds can be advantageously obtained.

  Furthermore, the lifetime of free carriers in the chip 42 determines the maximum fall time of the electrical pulse. Therefore, the fall time period as well as the exact period of the pulse is controlled by complementary means. However, the free carrier annihilation rate may be increased by applying a second light pulse to the chip 42 in certain applications, such as the application shown in FIG. 12 and described in the following description. The second light pulse has a wavelength ν ′ that is different from the wavelength used to set the chip to a conductive state. The purpose in this case is to obtain an electrical pulse having a trailing edge with a period on the order of the leading edge period.

  Furthermore, the wavelength of the light pulse 45 is selected as a function of the material used to produce the photoconductive effect. The light intensity of the light source 43 used should also be selected to sufficiently reduce the electrical resistance of the chip when the chip is in a conductive state (closed switch).

  Thus, due to the size of the generated pulses and the degree of gradient of the leading and trailing edges that can be obtained, the device according to the invention, particularly in the preferred embodiment described above, is an electrical pulse generator conventionally used for atom probes ( Advantageously replace (see FIG. 3).

  Furthermore, by using a chip of semiconductor material as a switch, the switching part of the device thus fabricated can be integrated directly into the vacuum chamber of the probe, so that the switch It can be advantageously placed in the immediate vicinity. The adaptation of the switching circuit and thus the shape of the generated pulses can thus be better controlled.

を有する。したがって、本発明による装置は、どんな共振振動も回避するために、前縁および後縁がこの値より大きな期間を有するパルスを生成できるようにするべきである。 A variation of the device according to the invention in a preferred embodiment for use as an evaporative pulse generator in an atom probe is extensive and includes the placement of the electrode 32 relative to the sample 33 and the generated electrical pulse to the electrode 32 and sample 33. It depends in particular on the method of making the overall electrical connection 46 to allow transmission. Various alternative embodiments are shown as non-limiting examples in the following description. These alternative embodiments specifically consider the following facts. That is, as shown by FIG. 6, the assembly formed by the back electrode and the sample tip constitutes an RLC type circuit whose resistance R _p varies from approximately zero ohms to 1000 megohms, and its capacitance C _p and inductance Consider the fact that L _p is on the order of 100 picofarads and 1 nanohenry, respectively. This circuit has a resonance frequency ν _P whose characteristic time is about 1 to 5 picoseconds.

Have Therefore, the device according to the invention should be able to generate pulses with leading and trailing edges having a duration greater than this value in order to avoid any resonance oscillations.

  Figures 7 to 10 show a first variant of the device according to the invention as well as the general operating principle of the invention.

  FIGS. 7 and 8 schematically show a first embodiment of the device according to the invention and allow its operation to be detailed. This first embodiment is, of course, presented as a non-limiting example, as is the case with other embodiments shown in the following description.

In this alternative embodiment, the voltage generator 71 of impedance R ₀ is connected to the ring electrode 73 by a transmission line 72 of length L ₀ . The electrode is placed facing the end of the sample 76 to be analyzed, but this sample has a tip shape and is itself at potential V ₀ .

The semiconductor chip 75 is disposed at the base of the ring 73, and the ring 73 is connected to a load impedance Z _c , 74 that closes the line 72.

Propagation line 72 has a characteristic impedance equal to load impedance Z _c , 74, typically on the order of 100 ohms, while the value of resistance R ₀ of generator 71 maximizes voltage wave reflection. Selected as Furthermore, the value of R ₀ is less than the impedance R _off of the chip 75 in the non-conductive state, typically several megohms, so as to obtain the maximum voltage across the terminals of the chip. Therefore, various impedance relationships are set as follows.
R ₀ >> R _off >> Z _c

  Field effect evaporation of some atoms in sample 76 occurs when ring 73 initially at a potential close to the reference potential is made negative with respect to this reference. Control of the evaporation process, which affects the resolution of the probe, requires that this potential be applied to the ring for only a very short time, so that only a few atoms are evaporated.

FIG. 7 shows the device in the absence of stimulation. The semiconductor chip 74 is then non-conductive, so that it has a high impedance R _off . Next, the ring 73 is set to a potential substantially equal to the reference potential, for example, ground potential (V _ring ˜0). In this way, since sample 76 is only brought to potential V ₀ , no evaporation occurs.

On the other hand, FIG. 8 shows the device in the presence of a stimulus. This stimulation takes the form of a light pulse, which triggers the device by making the chip 75 of semiconductor material conductive, at which time the chip 75 is low or emergency for a given time. to have a low value of resistance R _on the.

Therefore, the variation in the value of the resistance R of the tip 75, whereas the direction of the generator 71 is, in the direction of the load Z _c, 74 on the other hand, the voltage value V propagating along the line at a velocity V _S Step 82 Leads to the appearance at time t ₀ . Considering that the impedance of the chip 75 is approximately zero and the value of the load impedance 74 and the characteristic impedance of the line 72 is approximately equal to the same value Z _c , the voltage step amplitude V is approximately equal to −V _p / 2. . Furthermore, the rise time τ of this step is a function of the time spent by the material to become conductive under the influence of the light pulse. It is about 1 picosecond.

Upon reaching the input of the generator 71, the voltage step 83 is reflected by R _0, a high value of resistance R ₀ for values of impedance Z _c background, with a substantially equal amplitude and V p _{/ 2} Generate step 84 of opposite sign, this time step 84 propagates along line 72, resulting in 2 · L ₀ / vs (vs represents the propagation speed of the signal along line 74) After approximately equal time Δt, the incident step 83 and the reflection step 84 are summed in the ring 73. Next, the voltages applied to the ring 73 cancel each other at a time (fall time) equal to the rise time. Thus, due to the length L _{0 of} line 72 and the fact that the ring is located near tip 75, ring 73 first receives a voltage step V with a rise time τ and then a time Δt _0. At the end of, an opposite voltage step -V is received, which also has a fall time equal to τ. Accordingly, a pulse of period Δt ₀ having an amplitude −V _p / 2 and a leading and trailing edge of period τ is applied to ring 73.

Note that advantageously, the values of parameters τ and Δt ₀ defining the shape and duration of the pulses applied to ring 73 can be controlled. Specifically, the leading and trailing edge periods τ are directly a function of the semiconductor material used and the intensity of the applied light pulse 81. Next, the period Δt ₀ is determined by the nature and length of the transmission line 74.

Figures 9 and 10 show the advantageous effect of using the device according to the invention by means of simulation results. For this simulation, the semiconductor chip 75 is modeled _on an RLC circuit, ie an RLC circuit whose resistance R _on under laser irradiation is equal to 10 ohms and whose resistance R _off without irradiation is equal to 1 meg ohms. The inductance L is set here to 5 · 10 ⁻¹² Henry and the capacitance C is set to 10 ⁻¹³ farads. Further, the voltage _{V p} to be delivered by the generator 71, is set to 1000 volts, the resistance _{R 0} is set to 33 kohms. Further, the impedance Z _c is set to 100 ohms, the length of the transmission line 72 is set to 2 centimeters. Finally, the propagation velocity along line 72 is set to 200 meters per microsecond, and the carrier generation time in the semiconductor making up chip 75 is here set to 2 picoseconds.

The oscillogram of FIG. 9 shows that when such a device is used when a semiconductor chip is illuminated by a light pulse, it has a period approximately equal to 200 picoseconds, and a very short leading edge 92 and a rear of an equal period of the order of a few picoseconds. It shows that an electrical pulse 91 with an amplitude close to the edge 93 as well as V _p / 2 is generated. Also, advantageously, it can be seen that the duration of the generated pulse depends only on the propagation time of the voltage step along the propagation line 72.

  The illustration of FIG. 10 shows mass spectra 101 and 102 respectively obtained by using a pulse generator of the well-known type according to the prior art and a pulse generator produced by the device according to the invention as shown by FIGS. A single view is shown for a uniform sample of elements with a mass equal to 30 AMU. As shown in the figure, advantageously, by using the device according to the invention, it is possible to generate a pulse, ie a pulse whose characteristics improve the overall performance of the atom probe integrating this device. Become. From a mass resolution point of view, this improvement is demonstrated in the figure by a base attenuation of the spectrum of at least an order of magnitude relative to the spectrum of the same sample obtained by using known conventional means.

  FIG. 11 schematically shows a second alternative embodiment of the device according to the invention.

In this variation, the evaporative pulse generator is configured such that the sample to be analyzed is incorporated directly into the assembly rather than the ring electrode 73. The potential reference of the device is made the potential V ₀ applied only to the sample in the previous embodiment, so the assembly consists of a common potential device and sample equal to V ₀ .

Furthermore, the voltage generator is configured to deliver a voltage + V _p which can be added to the voltage V ₀ applied to the sample.

Thus, in the outage, ie in the absence of irradiation, the chip 74 of semiconductor material is then non-conductive, so that it has a high impedance R _off . Next, the sample 76 is brought to a potential approximately equal to the reference potential V ₀ , so that no melting occurs.

On the other hand, during operation, i.e. the chip 74 of the semiconductor material is exposed to radiation, if it has a very low impedance R _on therefore, sample 76, sample 76 over the entire duration of the voltage pulse to V _{0 +} V _p Subjected to a voltage pulse that brings the potential to approximately equal, this causes the desired evaporation.

  As in the previous embodiment, in this second alternative embodiment, the duration of the pulse is determined by the length of line 72 that allows the voltage delivered by generator 71 to be applied to sample 76. Therefore, by using this second configuration in an atom probe, the same advantages as obtained with the previous configuration are obtained in terms of resolution.

  FIG. 12 schematically shows a third alternative embodiment of the device according to the invention.

  The deformation described here differs from the previous deformation in that the period of its leading and trailing edges is obtained simply by affecting the chip 75 of semiconductor material, as well as the duration of the electrical pulses generated. .

The chip 75 is rendered conductive (R = R _on ) by applying a first light pulse 121, and then a second light pulse 122 with a wavelength different from the first light pulse is applied for a time Δt. _By applying at the end of ₀ , it is again rendered non-conductive (R = R _off ). In this way, an electric pulse of the period Δt ₀ is generated.

  As described above, the operation of the first light pulse 121 is to generate photoinduced free carriers in the semiconductor material. On the other hand, the second light pulse 122 affects the semiconductor by activating the semiconductor recombination sites so as to cause almost instantaneous destruction of free carriers. According to this embodiment, the semiconductor material making up the chip is selected to respond to the two wavelengths in question.

According to the present invention, the time interval Δt ₀ between the two optical pulses 121 and 122 is generated by any known means, for example an optical delay line.

  The time period is a function of the time interval between two light pulses, and the leading and trailing edges are time spent to make the semiconductor conductive and the free carrier recombination generated by the first light pulse. An electrical pulse with a duration of the order of a few picoseconds, which depends only on time, is thus obtained.

Claims (7)

A tomographic atom probe having means for applying an electroevaporation pulse of amplitude V _p and duration Δt ₀ to a sample at potential V ₀ , these means comprising:
- is the initial potential V _i, and an electrode that is configured and arranged to apply said electrical pulse to the sample,
A voltage generator capable of generating the voltage required to generate a pulse of amplitude V _p , connected to said electrode by an electrical connection that can be opened or closed;
Means for closing the electrical connection at a given time τ to apply a voltage step _Vp to the electrode, the means being close to the electrode, the generator and the A chip of semiconductor material disposed at the electrical connection between the electrodes and a first light source that emits a light pulse of wavelength λ _{1 to} the semiconductor chip, the chip having a wavelength λ ₁ Means that become conductive when irradiated with a light pulse of, closing the electrical connection, and the conduction time is a function of the duration of the applied light pulse;
Means for applying a voltage step of amplitude (−V _p ) to the electrode at the end of time Δt ₀ after closing the electrical connection to bring the electrode to the potential Vi _; Means for applying the voltage step at a time τ ′ approximately equal to τ;
A tomographic atom probe characterized by comprising: After closing the circuit, at the end of time Δt ₀ , the means for applying a voltage step of amplitude −V _{p to} the electrode comprises a characteristic impedance Z _c that connects the generator to the chip. The tomographic atom probe according to claim 1, comprising a transmission line terminated downstream of the electrode with an impedance equal to Z _c . The length L _{0 of the} transmission line is determined by the value of the time interval Δt ₀ in question, which time interval is equal to the duration of the generated electrical pulse. Fault atom probe. After closing the circuit, at the end of time Δt ₀ , the means for applying a voltage step of amplitude −V _{p to} the electrode emits a light pulse of wavelength λ ₂ onto the semiconductor chip. a light source, the chip, it becomes insulating when irradiated with a wavelength lambda ₂ of the light pulses, characterized in that it comprises a second light source to open the electrical connection, according to claim 1 The tomographic atom probe described in 1. 5. The time interval Δt ₀ between the emission of an optical pulse with wavelength λ ₁ and the emission of an optical pulse with wavelength λ ₂ determines the duration of the electrical pulse to be generated. The tomographic atom probe described.   The tomographic atom probe according to claim 1, wherein the electrode from which the electric pulse is generated is located facing the sample end where the evaporation is expected.   The tomographic atom probe according to claim 1, wherein the electrode from which the electric pulse is generated is formed of the sample itself.

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